Instantaneous automatic gain control for pulse circuits



- April 2, 1957 Y c. c. CUTLER 2,787,673

msmummaous AUTOMATIC GAIN CONTROL FOR PULSE cmcu'rws Filed on. 26, 1951 26 24 N0/V-L/NEAR //6 EXPAND/N6 oswc:

' 20 FIG. I 28 A TTENUA TOR 22 NON-LINEAR 's 72%, MM LIMIT/N6 I ATTENUATOR DEV/C5 Q /4 (MOW ourPur V2 fi ATTORNEY lNSTANTANEOUS AUTOMATIC GAIN CONTROL FOR PULSE CIRCUITS Cassius C. Cutler, Gillette, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 26, 1951, Serial No. 253,433

3 Claims. Cl. 179-171 This invention relates to automatic gain control circuits and more particularly to gain control circuits for amplifiers for the transmission of periodic pulse signals.

It is often desirable in amplifiers for periodic pulse signals to provide automatic means for varying the gain of the amplifier to provide maximum amplification for desired pulses and to suppress the transmission of undesired noise or other signals during the interpulse intervals. A gain control characteristic of this type is substantially the inverse of the usual automatic gain control characteristic employed in continuous wave amplifiers wherein the gain is increased for input signals of low amplitude and decreased for input signals of large amplitude to maintain a substantially constant output level.

It is an object of the present invention to provide gain control means for periodic pulse amplifiers which will meet the special requirements set forth above. It is a further object of the invention to provide such gain control arrangements for traveling wave tube amplifiers for the transmission of pulses at microwave frequencies.

In accordance with the present invention there is provided a form of instantaneous automatic gain control in which the gain of the amplifier for each pulse is adjusted in response to the amplitude of the immediately preceding pulse and the transmission of signals is substantially prevented during the pulse intervals (the intervals between pulses). In this arrangement the output of the signal amplifier is applied to a pair of impedances having differing input-output characteristics which coincide only at a single amplitude level. The outputs derived from these impedances are combined in phase opposition and the difference is applied, after amplification it necessary and the introduction of additional delay sufiicient to make the total delay between the signal applied to the amplifier and the control wave equal to the interpulse interval, to adjust the gain of the signal amplifier.

The above and other features of the invention will be described in detail in connection with the drawings in which:

Fig. 1 is a block diagram of an instantaneous automatic gain control circuit according to the invention;

Fig. 2 is a schematic diagram illustrating the application of the invention for controlling the gain of a traveling wave tube amplifier operating at microwave frequencies; and

Fig. 3 is a schematic diagram illustrating a modification of the invention for use in controlling the gain of an amplifier for direct current pulses.

The invention may be understood by reference to the block diagram of Fig. 1 in which the signal amplifier is indicated at 10. This may be any type of amplifier having a frequency response suitable for the intended use and the operating potentials for the amplifier are normally chosen to provide relatively low gain. In accordance with the invention, then, a control voltage is required which is of the proper polarity to increase the gain of the amplifier at the time of the next expected pulse.

2,787,673 Fatented Apr. 2, 1957 In a conventional remote cut-oft" amplifier, which is normally biased for low gain, a positive control voltage may be applied to the gain control element to increase the:

gain of the amplifier at the desired time. In order to obtain such a control voltage the output of the amplifier is applied over lead 12 to an output circuit and also over lead 14 to the output of two impedance elements 16 and 18 shown in Fig. l as constituting respectively a nonlinear device having an expansion characteristic and a non-linear device having a limiting characteristic. It is to be understood however that impedances 16 and 18 need not necessarily have the particular characteristics indicated in Fig. 1. For example, one of the impedances may have a linear characteristic while the other has a limiting characteristic. One may be a linear detector or rectifier and the other a square law detector. The only requirement is that the two impedances have substantially different input-output characteristics. Attenuaators 20 and 22 located respectively in the input connection to impedance 16 and in the output of impedance. 13 are provided and are so adjusted to produce equal. output potentials at the point 24 from the two impedance; devices at a particular signal level. According to the; invention, attenuators 20 and 22 and the characteristics of the two impedances are so adjusted as to provide equal output voltages in response to the maximum signal amplitude. As stated above, it is necessary only that the two impedances have substantially diiierent characteristics which however coincide at only one level of applied signals. It will be recognized, therefore, that at any other level of applied signals, the outputs of the two impedance element circuits will be different and when added in opposition will produce a residual output voltage. Preferably the two characteristics or at least the portions thereof employed in the operation of the automatic gain control circuit of the invention should be such as to produce an increasing large difference output as the signal level departs from the maximum. These conditions are met by the expander and limiter characteristics indicated-as being employed in the circuit arrangement of Fig. 1. The particular locations of attenuators 26 and 22 shown in Fig. l are desirable when the impedance elements 16 and 18 respectively have expanding and limiting characteristics. Under these conditions, attenuator 20 reduces the input level at the expander so that it may operate at greatest efliciency without depriving the limiter of the full signal amplitude. Similarly, attenuator 22 permits adjustment of the limiter output to fall in the desired range. It will be understood that a d-itferent arrangement of attenuators may be preferable if the impedance elements have other characteristics.

The outputs from impedances 16 and 18 are combined? in phase opposition and the resulting difference voltage. is applied to an amplifier 26. This amplifier may have:

a small band width relative to the band width of the signal amplifier 10 and may have, for example, a band. width less than three times the pulse repetition rate there-- by to eliminate the transmission of harmonics of this: frequency to the gain control element of the signal. amplifier.

The operations performed upon the control signals by the circuits thus far described introduce a certain amount of phase delay which in general will not be equal to the pulse interval of the signals applied periodically to signal amplifier 10. Accordingly, the output signal from amplifier 26 is applied to a phase shifter or delay circuit 28 which introduces sufficient delay to make the total delay between the signal applied to amplifier 10 and the control signal applied to control the gain of amplifier 10 substantially equal to the pulse interval.

In the operation of the circuit of Fig. 1, whenever a to derive a control signal which isapplied to the 'coiitro-l element" of the signal amplifier to provide maximiurrgain at the approximate time of the next expected pulse of the periodic pulse wave. It will be recalled that the.in-. put to amplifier 26 resulting from the action of the non linear elements 16 and 13 is at theminiinum. for maximum amplitude of the output ofsignal amplifier 1-2. This may be made of the proper characteristics for the required control voltage by phase inversion in amplifier 26. Thus for maximum signal amplitude oi the output of amplifier l0, thecontrolvoltageat'the outputof amplifier 26is at'its maximum value and may he applied i after an appropriate phase delay introduced by phase shifter 28 to increase the gain of amplifier 1t for'the' next pulse. the time of arrival of the pulses of a periodic pulse train and the amplifi cationof noise signals between these times is suppressed.

The application of the invention to a microwave pulse amplifier employing a traveling wave tube is illustrated in Fig. 2. Here the travelingwave amplifier. includes a cathode 32, a control grid 34-, a helix 36, and a collector Thereafter maximum gain is available at 38 together with wave guide input and output circuits it and 42 respectively. Such travelingwave amplifiers are described in detail in the Proceedings of the institute of Radio Engineers for February 1947 in the following articles: Traveling Wave Tubes by I. R. Pierce and L. M. Field, pages 108 through 111; Theory of thc Beam Type Traveling Wave Tubes by J. R. Pierce, pages ill through 123; and The Traveling Wave Tube as Amplifier at Microwaves by R. Kompfuer, pages 124 through 127. Appropriate operating potentials are applied between the cathode and helix at the cathode and collector by a suitablepower supply shown herein schematically as a battery 40.

Output signals from the traveling wave tube are applied through output wave guide 42 to the input arm of a hybrid junction 44. The arm conjugate to the in-' put arm of the'junction is terminated by a matching im-. pedance 46 while the signal output is obtained from one of the remaining pairs of arms. The fourth arm is connected to a wave guide T 47 across the respective arms of.

which are connected crystal rectifiers 48 andStl 'corresponding to impedances 16 and 18 of Fig.1. It is well known that non-linear devices such as crystal rectifiers present essentially square-law characteristics at low'lamplitude levelsand linear or at least Widely difierent char acteristics at higherlcvels. Thus rectifier 50 which opcrates at low levelsbecause of attenuator 52 and rectifier 48 which operates at high levels present the required ditferent characteristics. As shown in Fig. 2 these rectifiers may be'connected to the wave guide arms in which they are mounted through capacitors which complete the high frequency circuits without causing a direct-current short circuit. Alternatively the crystal elements may be mounted in holders supported externally of the wave guide and provided with probes extending into the wave guide. I provided in the arnr'of the T in which crystal 59 is mounted while an attenuator 54 is connected in the output circuit of the other crystal rectifier. The outputs from c'rysals 43 and 50 are applied in opposition between the cathode and grid elements of a conventional triode amplifier 56.

The output of the amplifier, appearing across load resistor 58 connected in series with power supply battery 60 between the anode and cathode of tube 56, is applied to a phase shifter indicated herein as comprising a delay line having series inductors 62' and shunt capacitors 64o The output of this delay line is of the proper phase to correspond to the time of the next expected inputpulse An attenuator indicated schematically at 52 is and is applied between the control grid 34 and the cathode 32 of a traveling wave tube and increases the gain thereof for the incoming pulse, the phase reversal in amplifier 56 being efiective to provide'a control signal of. the necessary polarity as in the case of Fig. 1.

As illustrated in Fig. 3, ,the invention is also applicable to amplifier circuits for so-called direct current pulses.

As shown in thisfigure input pulsesare applied between the control grid and. cathode of a tetrodeamplifier tube 66, the anode circuit of which includesa load resistor and a battery 70; Theamplified pulseoutput from tetrode 66 is applied through acapacitorn to the input of a non-linear rectifier, the arms of which include rectifier elements 74,76, -78 and 80. "Of these elements, rectifiers '74 and 5t) arepoled as to presenthigh resistances while rectifiers 76 and 78 'are poled to present low resistances to signals applied to the input diagonal of the bridge. It will be understood therefore that the rectifier bridge is normally uiiblanc'edwith'th'e result thathn output appears betweenleads 82 and Ei l'c'onnected across the output diagonal of'the bridge. The bridge is balanced ata particular inputsigaariever and preferably af'the lei/cl corresponding to themaxirnum inputsignalto be accepted: b'yamplifier oo by connecting in shunt of rec tifie'r elements 74', .76, 73'and titl'resistors dd'which are so proportioned asto'hiake'the resistances in the four arms of the'bridg'e equal at the' c'hosen signal level. At any'othe'r input'level the bridge is unbalanced and an output appears between loads 32 and 84.

'The'output'of the, bridge is applied tothe input of a triode amplifier 88; output lead82 being connected to the grid and output lead 34 to the cathode thereof. The anode of amplifierfifi is connected'through the 131i mary winding of coupling transformer 90 to the positiv'e'terminalfof'anode supply'battery92, the negative terminalof'whichisconnected to the cathode of the tube. Aueiay line 94fc6rr'espoi1'ding to the delay line shown in Fig. 2is' cohnetda'cro'ss the secondary winding of transformer 90 and rheumatism the line is'applied as a gain control'volta'ge' infseries with bat't'er'y'96 between the screen grid and 'thecathodc of signal amplifier 66.

Operation of. the circuit-shown in Fig. 3 is analogous in all respects to that of the previously described circuits of Figs. 1 and 2', the' only differenc'e beingjthat no rectification .is r'e'ciuired in the automatic gain control loop since only direct 'c'urrent puls e'slappear at the output of signal amplifier 66f What isiclaiihedis.

1. Anaiitomaticgainicontrol circuit for a broad pulse amplifier arra'ng'ddo "t'r'aiisrriit periodic pulses of pedance,fele'nfentsv having characteristics which coincide said impedances'i'n 'phase'opp'os'itiom and means for ap" plying the output of said combining means to control the gain of said 'an'tpn e r; said last-'rri'entioned means introducing a phase ueu 'id make the total phase delay in the circuit includin g' said. impedance's and said combining means substantially quarre the purse interval.

'2. An automatic gain control circuit fora broad band amplifier for periodic pulses of fixed, pulse interval conipri'sing a pair of non-linear elements having impedance characteristics 'whichc'oiiicide at only one signal level. means for applying the output signalfromthe amplifier to each of said impedance's, means for combining the output of said impedauces in phase opposition, an amplifier for said combined outputs'having a'band width which is narrow as compared with the bandwidth of said pulse amplifier to produce an alternating current signal for application to control thegain of the pulse amplifier as a function of the str'eugth of'the amplified signals; and a delay element 'eonneered nsatu automatic gain control circuit to maketheYotal phase delay between the output of the pulse amplifier and the control signal applied thereto equal to said pulse interval.

3. An automatic gain control circuit comprising a traveling wave amplifier arranged for the transmission of radio frequency pulses and comprising an electron gun, a helix and a collector anode, a pair of non-linear impedances having impedance values which are the same in only a particular signal level, means for applying'the output pulses from said traveling wave tube to said impedances, means for obtaining a control signal proportional to the difierence between the outputs of said impedances, and means for applying said control signal to the electron gun of said traveling wave tube after a total delay equal to the pulse interval to vary the gain of said traveling wave tube.

References Cited in the file of this patent UNITED STATES PATENTS Doba Oct. 26, 1937 Weber Nov. 8, 1938 Wheeler Aug. 20, 1940 McCarty Dec. 15, 1942 Scherbatsloy Sept. 14, 1943 Barney Feb. 22, 1944 Gillespie Apr. 4, 1944 Somers Nov. 28, 1944 Herbst Oct. 21, 1947 Loper May 23, 1950 Graham May 22, 1951 Hawkins et a1 May 29, 1951 Wolfe Feb. 12, 1952 

